1. Field of the Invention
The present invention relates to a strain sensor of a type generally useable in various load cells for detecting the weight of an object to be weighed or an accelerometer and, more specifically, to a method of manufacturing such strain sensor.
2. Description of the Prior Art
A load cell generally used in a weight detecting apparatus such as, for example, an electronic scale makes use of an elastic element of a generally hollow rectangular configuration comprising stationary and movable rigid bodies defined at opposite ends thereof and upper and lower beams extending between the rigid bodies while spaced a distance from each other, each of said upper and lower beams having a region of low rigidity. The stationary rigid body is fixedly secured to a support such as, for example, a housing framework of the apparatus and the movable rigid body is adapted to receive a load of which weight is desired to be measured. When in use the load is applied to the movable rigid body of the elastic element through, for example, a weighing table, the movable rigid body displaces relative to the stationary rigid body in a quantity proportional to the weight of the load, accompanied by the upper and lower beams yielding at the respective regions of low rigidity. By detecting a tensile strain or a compressive strain developed at the regions of low rigidity of the beams as a result of the relative displacement between the stationary and movable rigid bodies, an indication of the weight of the load applied can be obtained.
A sophisticated version of the load cell of a three-beam design is now conceived by the applicant. This prior art three-beam type load cell is shown in FIG. 11 of the accompanying drawings. Referring to FIG. 11 for the discussion of the prior art, the three-beam type load cell shown therein comprises an elastic element D of a generally parallelopiped configuration including rigid bodies A defined at opposite ends thereof, upper and lower beams B extending between the rigid bodies A in parallel relation to each other, each of said beams B having a pair of regions of low rigidity B1, and arms C extending from the respective rigid bodies A into a space, delimited by the rigid bodies A and the beams B, in alignment with each other in a direction close towards each other. A strain sensor E is fitted at its opposite ends to the respective arms C so that the elastic element D as a whole represents a three-layered structure. When in use a displacement of one of the rigid bodies A relative to the other of the rigid bodies A in a direction generally perpendicular to any one of the beams B results in a tensile strain or a compressive strain developed on the surface of a substrate of the strain sensor E, and a subsequent detection of the strain so developed on the strain sensor can provide an indication of the magnitude of the load applied, that is, the weight of an object having been weighed.
The strain sensor E employed is well known and is manufactured by the following manner which will now be described with reference to FIG. 12. Namely, a generally rectangular substrate F has its opposite ends adapted to be fixedly secured to the respective arms C of the elastic element D as shown in FIG. 11. This substrate F has two pairs of generally U-shaped notches F1 formed therein with each pair adjacent the corresponding end of the substrate E while the notches F1 of each pair extend inwardly of the substrate E from opposite side edges of the substrate E in alignment with each other so as to leave an associated neck region F2 having a low rigidity. A detecting element G having a pair of strain sensing areas G1 each comprised of a fine resistance wiring is deposited on an upper surface of the substrate E with the strain sensing areas G1 positioned respective surface portions of the substrate E where the associated neck regions F2 are defined.
It has however been found that, in the case of the strain sensor E of the structure described above, the amount of strain induced in each of the neck regions F2 of low rigidity as a result of application of a load, that is, the magnitude of an output from the detecting element G or the sensitivity to the strain depends on the ratio (L1/L2) of the width L1 of the substrate F relative to the width L2 of each of the neck regions F2 of low rigidity left by the respective pairs of the U-shaped notches F1. Accordingly, in order to increase the sensitivity to the strain, either must the width L1 of the substrate F be increased, or the width L2 of each of the neck regions F2 of low rigidity must be reduced. However, in view of the fact that the strain sensing areas G1 of the strain detecting element G are formed on the substrate F between the respective pairs of the U-shaped notches F1, that is, on the respective neck regions F2 of low rigidity, reduction in width L2 of each of the neck regions F2 of low rigidity is limited practically. Hence, a practical compromise to increase the sensitivity to the strain is to employ the substrate F having an increased width L1, and this in turn results in an increase in size of the strain sensor.
Also, the strain sensor E of the above described structure is prepared, as shown in FIG. 13, from a relatively large plate material H having one surface formed with a plurality of strain detecting elements G each having a pair of the strain sensing areas G1 and arranged in a matrix pattern, which strain detecting elements G are subsequently separated from each other by means of a laser beam cutting technique to provide the individual strain sensors E each being shown in FIG. 13. In such case, since two pairs of notches F1 must be formed for each resultant strain sensor E, a cutting line along which the plate material H has to be cut by means of the laser beam cutting technique to provide the individual strain sensors E is so complicated as shown by the phantom line in FIG. 13 and, therefore, the cutting involves a complicated and time-consuming procedure to such an extent as to result in a reduction in productivity of the strain sensors E.
There is an additional problem. A YAG laser is generally used during the cutting of the plate material H to provide the individual strain sensors E since the YAG laser is superior in that it brings about little thermal influence on the surroundings. However, during the formation of the notches F1, the laser beam is traversed in close proximity to the individual strain sensing areas G1 which are most sensitive and important areas and, therefore, there is no way to avoid a thermal influence on the strain sensing areas G1. Therefore, it often occurs that some of the resultant strain sensors E are found defective and/or are liable of a reduction in detecting accuracy.